Ring laser angular rate sensors, often also called ring laser gyros, are well known. One example of a ring laser angular rate sensor is U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference thereto. Present day ring laser angular rate sensors include a thermally and mechanically stable laser block having a plurality of interconnected formed cavities. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-looped path.
In operating a ring laser gyro it is important to maintain the laser beam current in each leg of the ring laser gyro between an anode and a cathode within a desired operating range such as, for example, about 0.15 ma to about 1.0 ma. In the prior art, large resistors called ballast resistors are employed to maintain stability of the plasma within the desired current range. Unfortunately, such ballast resistors tend to be very large resulting in a large amount of wasted power. Further, it is necessary to select these ballast resistors for each individual ring laser gyro out of a range of selectable ballast resistors. This selection or calibration of each ring laser gyro, results in higher production costs and less reliable current control than that which is provided by the present invention. Ballast resistors used in the prior art had to be carefully selected in order to match the current in both legs to within better than one percent (1%) in order to reduce bias characteristics in the ring laser gyro. Further still, current control circuits of the prior art required high voltages and wide bandwidth circuits in order to achieve a high performance ring laser gyro.
The present invention overcomes the deficiencies of the prior art by providing an active current control apparatus which does not require selected ballast resistors, uses conventional active elements and medium performance operational amplifiers, and yields a high performance ring laser gyro with no plasma oscillations over the entire operating range of desirable currents. Furthermore, through the use of a microprocessor based controller, the active current control apparatus of the invention maintains a high degree of accuracy and reliability in a ring laser gyro system application.
As a basis for designing the active current control apparatus of the present invention, design data was taken on a GG1320 model number ring laser gyro as manufactured by Honeywell Inc. of Minneapolis, Minn. The data taken was within the operating window of laser beam current with cathode current as a function of ballast resistor and with capacitance as a parameter. Since the 1320 model ring laser gyro operates in the negative resistance region of the current-voltage characteristic, stray capacitance near the anodes can significantly effect the operating window. Operating windows as a function of current were obtained for the regions wherein plasma oscillations occurred. Ballast resistors as low as zero ohms and capacitance less than 15 pF had a very small effect on the operating window. This data was useful in defining the requirements for high voltage and low capacitance semiconductor devices employed in the present invention.